vuejs-development

What this skill does

# Vue.js Development Skill

This skill provides comprehensive guidance for building modern Vue.js applications using the Composition API, reactivity system, single-file components, directives, and lifecycle hooks based on official Vue.js documentation.

## When to Use This Skill

Use this skill when:
- Building single-page applications (SPAs) with Vue.js
- Creating progressive web applications (PWAs) with Vue
- Developing interactive user interfaces with reactive data
- Building component-based architectures
- Implementing forms, data fetching, and state management
- Creating reusable UI components and libraries
- Migrating from Options API to Composition API
- Optimizing Vue application performance
- Building accessible and maintainable web applications
- Integrating with TypeScript for type-safe development

## Core Concepts

### Reactivity System

Vue's reactivity system is the core mechanism that tracks dependencies and automatically updates the DOM when data changes.

**Reactive State with ref():**
```javascript
import { ref } from 'vue'

// ref creates a reactive reference to a value
const count = ref(0)

// Access value with .value
console.log(count.value) // 0

// Modify value
count.value++
console.log(count.value) // 1

// In templates, .value is automatically unwrapped
// <template>{{ count }}</template>
```

**Reactive Objects with reactive():**
```javascript
import { reactive } from 'vue'

// reactive creates a reactive proxy of an object
const state = reactive({
  name: 'Vue',
  version: 3,
  features: ['Composition API', 'Teleport', 'Suspense']
})

// Access and modify properties directly
console.log(state.name) // 'Vue'
state.name = 'Vue.js'

// Nested objects are also reactive
state.features.push('Fragments')
```

**When to Use ref() vs reactive():**
```javascript
// Use ref() for:
// - Primitive values (string, number, boolean)
// - Single values that need reactivity
const count = ref(0)
const message = ref('Hello')
const isActive = ref(true)

// Use reactive() for:
// - Objects with multiple properties
// - Complex data structures
const user = reactive({
  id: 1,
  name: 'John',
  email: '[email protected]',
  preferences: {
    theme: 'dark',
    notifications: true
  }
})
```

**Computed Properties:**
```javascript
import { ref, computed } from 'vue'

const count = ref(0)

// Computed property automatically tracks dependencies
const doubled = computed(() => count.value * 2)

console.log(doubled.value) // 0
count.value = 5
console.log(doubled.value) // 10

// Writable computed
const firstName = ref('John')
const lastName = ref('Doe')

const fullName = computed({
  get() {
    return `${firstName.value} ${lastName.value}`
  },
  set(newValue) {
    [firstName.value, lastName.value] = newValue.split(' ')
  }
})

fullName.value = 'Jane Smith'
console.log(firstName.value) // 'Jane'
console.log(lastName.value) // 'Smith'
```

**Watchers:**
```javascript
import { ref, watch, watchEffect } from 'vue'

const count = ref(0)
const message = ref('Hello')

// Watch a single ref
watch(count, (newValue, oldValue) => {
  console.log(`Count changed from ${oldValue} to ${newValue}`)
})

// Watch multiple sources
watch([count, message], ([newCount, newMessage], [oldCount, oldMessage]) => {
  console.log(`Count: ${newCount}, Message: ${newMessage}`)
})

// Watch reactive object property
const state = reactive({ count: 0 })
watch(
  () => state.count,
  (newValue, oldValue) => {
    console.log(`State count changed from ${oldValue} to ${newValue}`)
  }
)

// watchEffect automatically tracks dependencies
watchEffect(() => {
  console.log(`Count is ${count.value}`)
  // Automatically re-runs when count changes
})

// Immediate execution
watch(count, (newValue) => {
  console.log(`Count is now ${newValue}`)
}, { immediate: true })

// Deep watching
const user = reactive({ profile: { name: 'John' } })
watch(user, (newValue) => {
  console.log('User changed:', newValue)
}, { deep: true })
```

### Composition API

The Composition API provides a set of function-based APIs for organizing component logic.

**Basic Component with <script setup>:**
```vue
<script setup>
import { ref, computed, onMounted } from 'vue'

// Props
const props = defineProps({
  title: String,
  count: {
    type: Number,
    default: 0
  }
})

// Emits
const emit = defineEmits(['update', 'delete'])

// Reactive state
const localCount = ref(props.count)
const message = ref('Hello Vue!')

// Computed
const doubledCount = computed(() => localCount.value * 2)

// Methods
function increment() {
  localCount.value++
  emit('update', localCount.value)
}

// Lifecycle
onMounted(() => {
  console.log('Component mounted')
})
</script>

<template>
  <div>
    <h2>{{ title }}</h2>
    <p>{{ message }}</p>
    <p>Count: {{ localCount }}</p>
    <p>Doubled: {{ doubledCount }}</p>
    <button @click="increment">Increment</button>
  </div>
</template>
```

**Component without <script setup> (verbose syntax):**
```vue
<script>
import { ref, computed, onMounted } from 'vue'

export default {
  name: 'MyComponent',
  props: {
    title: String,
    count: {
      type: Number,
      default: 0
    }
  },
  emits: ['update', 'delete'],
  setup(props, { emit }) {
    const localCount = ref(props.count)
    const message = ref('Hello Vue!')

    const doubledCount = computed(() => localCount.value * 2)

    function increment() {
      localCount.value++
      emit('update', localCount.value)
    }

    onMounted(() => {
      console.log('Component mounted')
    })

    return {
      localCount,
      message,
      doubledCount,
      increment
    }
  }
}
</script>
```

### Single-File Components

Single-file components (.vue) combine template, script, and styles in one file.

**Complete SFC Example:**
```vue
<script setup>
import { ref, computed } from 'vue'

const tasks = ref([
  { id: 1, text: 'Learn Vue', completed: false },
  { id: 2, text: 'Build app', completed: false }
])

const newTaskText = ref('')

const completedCount = computed(() =>
  tasks.value.filter(t => t.completed).length
)

const remainingCount = computed(() =>
  tasks.value.filter(t => !t.completed).length
)

function addTask() {
  if (newTaskText.value.trim()) {
    tasks.value.push({
      id: Date.now(),
      text: newTaskText.value,
      completed: false
    })
    newTaskText.value = ''
  }
}

function toggleTask(id) {
  const task = tasks.value.find(t => t.id === id)
  if (task) task.completed = !task.completed
}

function removeTask(id) {
  tasks.value = tasks.value.filter(t => t.id !== id)
}
</script>

<template>
  <div class="todo-app">
    <h1>Todo List</h1>

    <div class="add-task">
      <input
        v-model="newTaskText"
        @keyup.enter="addTask"
        placeholder="Add new task"
      >
      <button @click="addTask">Add</button>
    </div>

    <ul class="task-list">
      <li
        v-for="task in tasks"
        :key="task.id"
        :class="{ completed: task.completed }"
      >
        <input
          type="checkbox"
          :checked="task.completed"
          @change="toggleTask(task.id)"
        >
        <span>{{ task.text }}</span>
        <button @click="removeTask(task.id)">Delete</button>
      </li>
    </ul>

    <div class="stats">
      <p>Completed: {{ completedCount }}</p>
      <p>Remaining: {{ remainingCount }}</p>
    </div>
  </div>
</template>

<style scoped>
.todo-app {
  max-width: 600px;
  margin: 0 auto;
  padding: 20px;
}

.add-task {
  display: flex;
  gap: 10px;
  margin-bottom: 20px;
}

.add-task input {
  flex: 1;
  padding: 8px;
  font-size: 14px;
}

.task-list {
  list-style: none;
  padding: 0;
}

.task-list li {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px;
  border-bottom: 1px solid #eee;
}

.task-list li.completed span {
  text-decoration: line-through;
  opacity: 0.6;
}

.stats {
  margin-top: 20px;
  padding-top: 20px;
  border-top: 2px solid #eee;
}

.stats p {
  margin: 5px 0;
}
</style>
```

### Template Syntax and Directives

Vue u